LONDON, Aug 20 (Reuters) - Prime Minister David Cameron said he was shocked that a British citizen seemed likely to be involved in a video purportedly showing the beheading of a U.S. journalist by Islamist militants, but added that the man had not yet been identified.

"We have not identified the individual responsible on the video but from what we've seen it looks increasingly likely that it is a British citizen. Now this is deeply shocking," Cameron told broadcasters on Wednesday.

But he ruled out sending troops to step up Britain's military involvement in Iraq, which has so far been focused on delivering supplies to Kurdish forces fighting Islamic State jihadists and using jets to conduct surveillance.

"I've been very clear this country is not going to get involved in another Iraq war," he said. "We're not going to put combat troops, combat boots on the ground - that's not something we should do. We have a clear strategy, we should stick to that strategy." (Reporting by Kate Holton and WIlliam James; editing by Stephen Addison)